In the yeast, Saccharomyces cerevisiae a method has been uncovered for selecting for deletions, duplications and transcriptions of a specific segment of chromosomal DNA which has been designated the COR region. In normal laboratory strains of this yeast there are two COR types, COR1 and COR2. Strains which are COR2 give deletions, duplications and transpositions of the COR region whereas COR1 strains do not. The COR region contains genetically identifiable loci, CYC1, OSM1 and RAD7, which code for iso-1-cytochrome c, osmotic sensitivity, and sensitivity to UV-irradiation. COR1 has been cloned using recombinant DNA procedures. Preliminary characterization, using the cloned COR1 DNA as probes, has shown that COR2 contains approximately 7 kb of contiguous material not present in COR1 and that several of the deletions and a transposition have one endpoint in this COR2 unique region. It is the purpose of this project to investigate the mechanisms involved in the formation of chromosomal aberrations in eukaryotic organisms using this system. COR2 will be cloned so that its structure can be compared to COR1. In addition, several of the deletions, transpositions and other chromosomal aberrations will be cloned and compared to the wild type COR structure. Also, mutants will be isolated that are deficient in the ability to give COR2 derived deletions. These mutants together with a comparison of the physical structures involved will help to elucidate the mechanisms involved in chromosomal rearrangements in eukaryotic organisms.